store consists of md5 content-addressed files

store file contents:
"AMEBAK" [version byte = 0] [compression mode byte] [file contents]

compression mode:
  'r' - uncompressed
  'F' - deFlate

compression should be attempted for all files, algorithms tested with
the first 10MB of data. if the reduction is <5%, do not compress. this
should make large high-entropy files have less overhead.

directory contents:
  store/ - see above
  index.db - list of backup files

backup files are stored in the store, and indexed in index.db. they are
archive files, with the contents of each file replaced with a binary
hash.